1. Field of the Invention
The present invention relates to a thermal recording apparatus for recording on a recording medium with heat, and more particularly to such a thermal recording apparatus adapted for use in an electronic typewriter, a word processor, a facsimile machine, a copying machine, a printer or the like.
The thermal recording apparatus includes a serial type in which a recording head is moved in the transversal direction of a recording sheet, a line print type in which a line is recorded collectively, and a page print type in which a page is recorded collectively. Also the recording method includes a so-called thermal transfer method in which ink is transferred from an ink ribbon for example onto plain paper, and a so-called thermal print method in which a thermal recording sheet is directly heated with a thermal head.
2. Related Background Art
FIG. 1 is a schematic plan view showing a principal portion of a conventional thermal recording apparatus.
A thermal head 3 is fixed on a portion of a head arm 31 facing a platen 11. The head arm 31 is approximately T-shaped, and the left end portion is rotatably supported by a shaft 32 on a carriage. From the center of a horizontal portion of the head arm 31 there vertically extends an arc-shaped rack 35 which meshes with a motor pinion 36. At the right-hand end of the horizontal portion of the head arm 31 there engages an end of a torsion spring 34, of which the other end engages with a hole 33 formed in the carriage 13. The torsion spring 34 is so provided as to bias the right-hand end of the head arm 31 toward the platen 11.
When the motor pinion 36 rotates clockwise in the illustration, the heat arm 31 rotates counterclockwise about the shaft 32, thereby pressing the thermal head 3 against the platen 11 across the recording sheet 1. In this state, the torsion coil spring 34, being so provided as to bias the head arm 31 toward the platen 11, remains pressed by the motor pinion 11 and does not hinder the printing operation.
On the other hand, in the non-printing state (head-up state), the motor is reversed to rotate the motor pinion 36 anticlockwise, thereby rotating the head arm 31 clockwise. The head arm 31 is returned against the biasing force of the torsion coil spring 34, and remains in the returned position since the biasing force of the torsion spring 34 is not applied beyond a certain angle.
However, in such conventional thermal recording apparatus, since the head pressure at the printing operation is constant, the pressing force cannot be regulated according to the kind of the recording sheet (surface smoothness, rigidity, thickness, etc.) though such regulation is in fact necessary. Consequently the pressure tends to be selected unnecessarily high, thus eventually reducing the service life of the thermal head. Also for the same reason, the electric power supplied to the electrothermal converting elements of the thermal head is often more than necessary, in order to increase the amount of generated heat, thereby reducing the service life of the thermal head. Also it is often not possible to obtain a clear recorded image.